1) Field of the Invention
This invention relates to a gas turbine using steam for cooling a high temperature member, a driving method of the gas turbine and a gas turbine combined electric power generation plant.
2) Description of the Related Art
At present, in order to enhance thermal efficiency in a gas turbine combined cycle, there is an increasing popular technique in which steam is used as a cooling medium instead of air, thereby cooling, by steam, a high temperature part such as a moving blade or stationary blade of the gas turbine. Here, a low pressure specific heat of dry steam is cp=1.86 kJ/kgK under a standard state, which is about two times of a low pressure specific heat of air (cp=1.00 kJ/kgK). Therefore, thermal capacity is greater and endothermic effect is higher as compared with air of the same mass as steam. Further, when wet steam is utilized as the cooling medium, wet latent heat of vaporization can also be utilized for cooling and thus, endothermic effect is enhanced. When steam is used as the cooling medium in this manner, it is possible to enhance the cooling efficiency as compared with a case such that air is used. Therefore, it is possible to increase a temperature of combustion gas around a turbine inlet and as a result, it is possible to enhance the thermal efficiency.
Conventionally, air from a compressor is used for cooling a moving blade and a stationary blade of a turbine, but when the compressed air is used for cooling the blades, working power which can be taken out from the turbine is reduced. When steam is used instead of air, cooling air for the moving blade and the stationary blade can be omitted, the working power which can be collected by the turbine is increased correspondingly, and the electric power generating efficiency is enhanced.
FIG. 14 is a partial sectional view of a gas turbine in which steam cooling is applied to the moving blade and the stationary blade. FIG. 15 is a schematic diagram which shows the gas turbine combined plant in which the steam cooling is employed for a high temperature part. In this gas turbine combined electric power generation plant, thermal energy of exhaust gas of the gas turbine is recovered by an HRSG (Heat Recovery Steam Generator: exhaust heat recovery boiler) 370. Steam is generated by the thermal energy of the exhaust gas of the gas turbine. This high temperature and high pressure steam is first supplied to a high pressure steam turbine 350 to drive the same, and electric power is generated by an electric generator 355 connected to the high pressure steam turbine 350.
Steam which worked in the high pressure steam turbine 350 is introduced into a moving blade 321 through a steam supply pipe 311 provided in the turbine main shaft 310 of the gas turbine. Further, steam is supplied to a stationary blade 325 from a steam supply port 330 provided outside a casing of the gas turbine. Here, cooling flow passages are provided in the moving blade 321 and the stationary blade 325. Steam introduced into the moving blade 321 or the stationary blade 325 absorbs heat of the combustion gas from an inner wall surface of the flow passage while the steam passes through the cooling flow passage, and the steam is discharged out from the flow passage. Then, steam which is already used to cool the moving blade 321 passes through a steam recovery pipe 312 provided in the turbine main shaft 310, and steam which is already used to cool the stationary blade 325 is discharged out from the gas turbine through the steam recovery port 331.
This cooling steam is introduced into a mixer 360 where the steam is mixed with cooling steam which is already used to cool a combustor receiver or the like and then, the steam is used as working fluid for driving a intermediate pressure steam turbine 351 or a low pressure steam turbine 352. Steam which is already used to drive the intermediate pressure steam turbine 351 or the low pressure steam turbine 352 is returned to water and then is supplied to the HRSG 370 by a pump, and the process is repeated again.
In the case of a conventional gas turbine in which steam cooling is applied to a high temperature member such as the moving blade and the stationary blade, for a while after start of the turbine, a portion of combustion air sent from a compressor is used to warm a rotor disk or the moving blade. This is because that when the moving blade, the stationary blade or the rotor disk is cooled by high temperature and high pressure steam immediately after the start of the gas turbine, thermal shock is generated by an abrupt temperature difference, and a problem is caused in such member sometimes. At the worst, such a member is damaged, and the gas turbine can not be driven. When air used for warming up is switched to steam and there exists a temperature difference between the air and the steam, however, the thermal shock is caused in the moving blade or the stationary blade to cause a problem there in indifferent degrees sometimes.
To solve this problem, Japanese Patent Application Laid-open No. 10-18809 discloses a technique in which air compressed is bled from an intermediate portion of a compressor, a high temperature portion of a gas turbine is cooled, and when air is switched to steam as the cooling medium, temperatures of the air and steam are controlled to eliminate the temperature difference. However, since the cooling air is bled from the intermediate portion of the compressing stage in the compressor, the driving state of the compressor becomes unstable due to stalling of the blade and as a result, cooling state of the high temperature portion also becomes unstable sometimes. Further, cooling air supplied to the moving blade becomes unstable and as a result, it becomes difficult to maintain a temperature of the high temperature portion constant, and the driving state of the gas turbine becomes unstable sometimes. It is difficult to maintain a stable driving state in this gas turbine, and a trip of the gas turbine is caused sometimes.
Each of the movable and stationary blades of a gas turbine is provided therein with a cooling flow passage through which cooling medium flows. A temperature of the moving blade and the stationary blade is not sufficiently warmed immediately after the driving of the gas turbine, and when the temperature thereof is lower than a temperature of saturated steam of the cooling steam, the cooling steam is condensed and becomes a drop of water. When the gas turbine is started after its operation is stopped, the cooling steam which flowed during the previous driving is condensed and becomes a drop of water and stays in the cooling flow passage sometimes.
Especially the moving blade rotates at high speed, and its rotation radius is great. Therefore, extremely large centrifugal acceleration as great as about 50000 to 100000 m/s2 is applied to the moving blade. Therefore, when a drop of water formed by condensing the steam exists in the cooling flow passage of the moving blade, the extremely great centrifugal acceleration is applied to the drop of water. Even when the amount of the drop of water is very small, the rotation balance of the gas turbine is largely broken to generate vibration, and at the worst, trip of the gas turbine is generated.
To solve this problem, there is conventionally used a technique in which air is allowed to flow through a cooling flow passage provided in each of a moving blade and a stationary blade before air is switched to steam as a cooling medium, cooling steam which also warm the moving blade or the like is condensed and becomes a drop of water, and the drop of water is purged. In order to obtain more complete purge, Japanese Patent Application Laid-open No. 11-93693 discloses a driving method of a combined cycle electric power generation plant in which after a high temperature part of a gas turbine is maintained in a vacuum state, a purge operation, a warming operation, a steam cooling operation and a stopping operation are sequentially carried out. Even when a drop of water completely remained in the cooling flow passage is purged, when the air is switched to steam as the cooling medium, vibration of a shaft in a rotor system of a gas turbine exceeds a permissible value sometimes. There still remains a problem that when the cooling medium is switched, trip of gas turbine is generated.
In a gas turbine in which the steam cooling is applied to a conventional high temperature member such as a moving blade, a stationary blade and the like, the moving blade and the stationary blade are independently provided with steam supply systems to cool the moving blade and the stationary blade. Therefore, a great amount of steam is required for cooling the moving blade and the stationary blade. Further, the speed of the gas turbine is increased after the start of the turbine and the gas turbine is brought into a constant speed (3000 or 3600 rmp) operating state. For a while after the gas turbine is started, rotation systems such as the moving blade, a turbine main shaft and the rotor disk are warmed. At that time, casing air supplied from a casing warms these parts and the casing air is about 400° C.
When the warming up is completed, the cooling medium of the moving blade is switched from the casing air into steam generated in the HRSG. At that time, it is necessary to uniform temperatures of the casing air and the steam so as to prevent the vibration of the shaft. At the time of the warming up operation, since a load of the gas turbine is low, a temperature of exhaust gas is about 300° C. Therefore, in the case of the casing air having a temperature of about 400° C., it is necessary to reduce its temperature by about 100° C. by means of a cooling unit before it is used for warming the moving blade and the like. Further, energy for cooling the casing air is required, and since the casing air is cooled before it is used, time is required for warming the moving blade and the like, and waste of fuel is generated correspondingly. In the gas turbine using the steam cooling system, there is a problem that a starting loss, i.e., cost capable of obtaining a rating output from the start of the gas turbine is increased.